1. Field of the Invention
The present invention relates to a high voltage electric appliance, more specifically, to an improvement of an insulating tape used for the high voltage electric appliance including electric coils which are wrapped by the insulating tape with subsequent thermosetting resin impregnation for electrical insulation.
2. Discussion of the Background
Built-up mica tapes of various types are widely used for electrical insulation of electric coils incorporated in a high voltage electrical appliance, e.g., a high voltage rotary machine, in light of their excellent properties of electrical insulation and resistance to heat. Generally, these mica tapes comprise built-up mica which is made by a so-called fourdrinier paper making machine from fine mica particles, and a reinforcing material, such as paper, a glass cloth or a polyester film, which is bonded to the built-up mica by an adhesive like a polyester resin, an epoxy resin or a silicone resin.
The fine mica particles forming the built-up mica are connected to each other by the water of crystallization contained in mica and the connection between the mica particles is frail. Due to such a frail connection, the built-up mica tends to peel off or tends to scatter, for example, in the process of wrapping the built-up mica tape around the electric coils for insulation. To overcome these problems, the amount of the adhesive is increased in an attempt to bond the mica particles. Alternatively, fibrids of aromatic polyamide are mixed with the mica particles in the paper making step so as to hold the mica particles with the fibrids in the built-up mica tape.
However, as the adhesive is increased in its amount, the mica content of the built-up mica tape is relatively decreased in the given thickness of the tape, which is undesirable in terms of the dielectric strength and heat resistance of the tape. Further, the increase of the adhesive content affects the resin impregnatability in the vacuum pressure impregnation treatment with a thermosetting resin for the wrapped electric coils of the high voltage rotary machine, for instance. In the case of admixture of aramide fibrids, the electric coils, when burnt for the purpose of reuse, generate harmful gases containing hydrogen cyanide, which has been an obstacle for such reuse of the electric coils or a bundle of conductors thereof.